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USMLE Renal > Physiology - Clearance, GFR, Calculations > Flashcards

Physiology - Clearance, GFR, Calculations Flashcards

1
Q

How much water is found intracellularly vs extracellularly?

A
  • 1/3 extracellular

- 2/3 intracellular

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2
Q

What percentage of water is interstitial vs plasma?

A
  • 1/4 Plasma

- 3/4 interstitial

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3
Q

What can inulin be useful for?

A

Determining GFR

- As it is neither secreted or resorbed

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4
Q

What space does inulin remain in?

A

Extracellular

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5
Q

What substance stays exclusively in the plasma

A

Radiolabeled Albumin

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6
Q

What is the normal value of plasma osmolarity?

A

~ 300mosm/kg

Equilibrium between cells and extracellular fluid

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7
Q

What kind of fluid causes an increase in both extracellular and intracellular fluid?

A

Hypotonic fluid

- E.g 5% Dextrose

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8
Q

What are the effects of Mannitol on fluid volumes?

A
  • Raises plasma osmolarity
  • Remains in the vascular system
  • Decreases ICF
  • Increases ECF
  • Reduces volume in interstitial space
  • Draws fluid out of brain cells/tissue into plasma
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9
Q

What is the effective circulating volume?

A
  • Extracellular fluid contained in arterial system
  • Maintains tissue perfusion (how well tissues are being perfused essentially)
  • Not necessarily correlated with total body water
  • Modified by: Volume, CO, Vascular resistance
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10
Q

What will a decrease in TPR have on the effective circulating volume?

A

Decrease effective circulating volume

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11
Q

What 2 main conditions cause a decrease in effective circulating volume but an increase in total body water?

A
  • Heart Failure

- Cirrhosis

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12
Q

What 2 main systems are activated by a low effective circulating volume?

A
  • Sympathetic NS

- RAAS

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13
Q

How is the GFR determined?

A

Determined from plasma, urine measurements

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14
Q

What is the Renal Blood Flow/ Plasma Flow?

A

How much blood enters kidney

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15
Q

What is the Filtration Fraction?

A

GFR/RPF

  • Can be used to determine how healthy kidneys are
  • Of all substance X entering kidney, what % gets filtered
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16
Q

What are the different methods of obtaining GFR?

A

Theoretical determination
- Need to know pressures in capillary, Bowman’s capsule

Clinical determination
- Need to know plasma concentration solutes, urine flow

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17
Q

What is the equation to calculate plasma volume?

A

TBV x (1-Hct)

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18
Q

What is a normal Filtration Fraction?

A

~ 20%

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19
Q

What is the effect of constriction of the efferent arteriole on GFR and RPF?

A
  • GFR will increase

- Renal Plasma Flow will decrease

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20
Q

What is the effect of constriction of the afferent arteriole on the GFR and RPF?

A
  • GFR decrease

- RPF decrease

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21
Q

What can dilate afferent arterioles?

A

Prostaglandins (NSAIDs = constrict)

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22
Q

What can constrict efferent arterioles?

A

Angiotensin II

thus ACEi will dilate efferent

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23
Q

Why would multiple myeloma decrease GFR?

A

Increases proteins in blood

  • Oncotic pressure in glomerulus increases
  • GFR decreases
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24
Q

How can an obstructed ureter cause decreased GFR?

A
  • Causes an increases in Hydrostatic pressure in Bowmans capsule
  • Decreased GFR
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25
What are the 2 mechanisms by which the kidneys autoregulate?
- Myogenic mechanism - Tubuloglomerular feedback Needed to maintain Renal blood flow and GFR to normal levels as BP fluctuates throughout the day
26
Describe the myogenic mechanism
Afferent arteriole constricts in response to high pressure - Responds to changes in stretch - Maintans normal GFR/RPF
27
Describe the tubuloglomerular feedback mechanism?
- Increased flow in tubule which increases NaCl to distal tubule - NaCl sensed by macula densa (part of JG apparatus) - Macula Densa -> vasoconstriction of afferent arteriole
28
What is the urine concentration?
Ux mg/L
29
\what is the urine flow rate?
l/min
30
What are the Measured Variables?
- Plasma Concentration (Px = mg/L) - Urine concentration (Ux = mg/L) - Urine flow rate (V = L/min) These measured variables are used to get RPF, GFR, etc.
31
What is the equation for Renal Clearance?
Cx = (Ux * V) / Px ``` Ux = Urine concentration V = Volume flow rate Px = Plasma conc ```
32
What are the units for Renal Clearence?
Litres/min (volume flow)
33
What is the clearence of inulin the same as?
GFR
34
Why does creatinine slightly overestimate GFR?
Approximate measurement | - Moderately secreted by renal tubules so slightly overestimates
35
What formula can used to estimate GFR? | What is taken into account in the calculation?
Cockcroft-Gault formula | - Encorporates age, weight, gender and Cr level to estimate GFR
36
What substance can be used to estimate Renal Plasma Flow (RPF)?
Para-aminohippuric acid (PAH) - Clearence of substance = Plasma to kidney - 100% of PAH that enters kidney leaves blood in urine - If it is not filtered through the glomerulus then it will be secreted by the nephron
37
Why is the true renal plasma flow slightly higher than that estimated by PAH?
Not all blood that goes to the kidneys is filtered - goes to other areas of kidney (underesteimates by ~ 10%)
38
When are NSAIDs avoided?
In patients with HF and Renal Failure
39
What does it mean if the amount filtered (filtered load) is the same as the amount of a substance excreted?
No secretion/resorption
40
What does it mean if GFR vis greater than nclearence?
Resorption must be occurring
41
What does it mean if GFR
Secretion must be occurring
42
What is the equation to work out Renal blood flow (RBF) from renal plasma flow?
RPF/(1-Hct) Hct is usually ~ 40% so usually RPF/0.6
43
What is a roughly normal Filtration Fraction?
~ 20%
44
What will the effect of prostaglandins be on RPF, GFR and FF?
- Increase RPF - Increase GFR - FF will remain normal
45
What will the effect of NSAIDs be on RPF, GFR and FF?
- Decrease RPF - Decrease GFR - FF will remain normal
46
What may be the clinical effects of NSAIDs on someone with bad kidneys?
- Acute renal failure | - Acute HF (accumulation of Na+ and water)
47
What are the effects of Angiotensin II on the RPF, GFR and FF?
- Increases GFR - Decreases RPF - Increases FF
48
What are the effects of ACEi on RPF, GFR and FF?
- Decreased GFR - Increased RPF - Decreased FF
49
What is the equation for calculating the amount of substance x being excreted?
Excreted (X) = V*Ux
50
What is the equation for calculating the amount of substance x being filtered?
GFR*Px
51
The amount excreted equals what?
Filtered - Reabsorbed + Secreted
52
What are examples of regulated solutes? (or solutes which do not have a concentration change in renal failure)
- Na+ (by ADH) | - K+ (by aldosterone)
53
What are the unregulated solutes in renal failure?
- Creatinine | - Urea
54
What is the filtered load?
Amount of substance x that is filtered into Bowmans space per unit time - Same as amount filtered
55
What happens to the filtered load of K+ and Na+ in renal failure?
Decreased
56
What happens to the levels of Na+, K+ and Urea that are excreted in renal failure?
The levels of the solute that are excreted remain the same as in normal kidney function - The amount excreted must = the amount taken in/ produced by body
57
What is fractional excretion?
Excretion / Filtered load
58
What happens to the fractional excretion of Na+ and K+ in renal failure?
Increased (excreting much more of what they are filtering, although are filtering less)
59
What happens to the fractional excretion of Urea in renal failure?
Remains the same
60
What is the clearence equation?
C = (U x V) / (P) ``` C = Clearence U = Urine conc. V = Urine flow rate P = Plasma conc. ``` Clearence of inulin = GFR
61
What is larger Renal Blood flow or renal plasma flow?
Renal Blood flow RPF/1-Hct
62
What is the equation to work out the amount filtered across the glomerulus (filtered load)?
GFR * Px
63
What is the equation for working out the amount of a substance excreted?
Excreted = Filtered - Reabsorbed + Secreted Filtered = GFR * Plasma conc

USMLE Renal (11 decks)

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